geojsonio: Convert Data from and to 'GeoJSON' or 'TopoJSON'

Convert data to 'GeoJSON' or 'TopoJSON' from various R classes,
including vectors, lists, data frames, shape files, and spatial classes.
'geojsonio' does not aim to replace packages like 'sp', 'rgdal', 'rgeos',
but rather aims to be a high level client to simplify conversions of data
from and to 'GeoJSON' and 'TopoJSON'.